1. Field of the Invention
The present invention relates generally to backlight assemblies for liquid crystal displays and the like, and more particularly to an improved total internal reflection light pipe for a backlight assembly.
2. Description of the Related Art
A standard backlight assembly for a liquid crystal display (LCD), such as for a lap top computer, is schematically shown in cross-section in FIG. 1. A backlight assembly 20 has a light propagating waveguide or total internal reflection (TIR) light pipe 22. The light pipe has one or more light sources 24 located along its edges. A commonly used light source is known as a cold cathode fluorescent light tube or CCFL tube and is placed along an edge or along opposed edges of the light pipe. The backlight assembly 20 also has a reflector or reflective layer 26 placed adjacent a back surface 28 of the light pipe. A number of optical elements 30 are disposed on the back surface of the light pipe.
In one known construction, the optical elements 30 are a plurality of silk screen, light colored dots that reflect light upward toward a front surface 32 of the light pipe. In an alternative known construction, the optical elements 30 are a plurality of elongate grooves formed in the back surface 28 of the light pipe 22 that direct light toward the front surface 32 at controlled angles. A separate diffuser film layer 34 is disposed adjacent the front surface 32 of the light pipe. The diffuser film 34 has a smooth back surface 36 facing the light pipe and a diffuser structure on a front surface 38 facing away from the light pipe.
A pair of brightness enhancing film (BEF(trademark)) layers 40 are located adjacent the diffuser film layer 34 and are oriented orthogonal relative to one another. THE BEF(trademark) layers are available from 3M of St. Paul, Minn. and BEF is a TRADEMARK OF 3M. One of the BEF(trademark) layers turns light in one direction or plane and the other BEF(trademark) turns light in another direction or plane 90xc2x0 relative to the first BEF(trademark) layer. Each BEF(trademark) layer typically has a plurality of optical elements 42 such as prisms located on a front surface 44. The BEF(trademark) layers together collimate light toward the direction normal to the light pipe front surface.
The backlight assembly also has a protective layer 50 disposed over the BEF(trademark) layers to prevent damage to the optical elements. A display panel such as a LCD panel (not shown) is typically placed adjacent the protective layer.
The silk screen dot density on the back surface of the light pipe determines the amount of light projected upward toward the front surface of the light pipe and determines uniformity of light distribution. Two alternative dot density distributions are shown in FIGS. 3 and 4 and are discussed below. The higher the density of dots in a particular region on the light pipe back surface, the more light that is projected upward from that region toward the front surface. The front surface of the standard light pipe is left essentially smooth or flat.
Such a standard silk screen dot backlight assembly produces a light brightness output exiting the backlight. FIG. 2 illustrates one possible light output representation. The HLP and VLP curves represent the light intensity or brightness for only light exiting the light pipe in a horizontal or a vertical direction, respectively, across the light pipe. The brightness is fairly consistent both vertically and horizontally across the light pipe. This is achieved by having a higher dot density near the middle of the light pipe back surface and a lower dot density near the light sources, as shown in FIG. 3. More light thus travels to the middle of the light pipe before being reflected out. The HBL and VBL curves illustrated in FIG. 2 show the brightness for the entire backlight assembly including all of the components. The diffuser film and two BEF(trademark) layers enhance the brightness near the center of the backlight where a viewer would be situated. The brightness near the edges of the backlight assembly is lower. The diffuser film and BEF(trademark) layers collimate the light and therefore direct more light towards the middle of the light pipe and upon exiting the light pipe, direct more of the light toward the normal and not at an angle relative to the normal. Therefore, a majority of the light and thus the greatest brightness is near the center of the screen.
FIG. 4 illustrates an alternative dot density distribution on the back surface 28 of the light pipe 22 where the dots are more dense near the edges adjacent the light sources 24. FIGS. 5 and 6 illustrate graphs representing the brightness intensity measured along the horizontal axis and vertical axis across the light pipe and the backlight assembly, respectively identified as curves HLP, VLP, HBL, and VBL. As can be seen, the brightness along the vertical axis for both the light pipe and the backlight follows a curve having a general xe2x80x9cMxe2x80x9d shape. This is caused both by the intensity being brighter nearer the light sources and the increased dot density nearer the sources. The brightness along the horizontal axis for the light pipe is fairly consistent across the surface. This is because the distance from the light sources and the dot density are consistent in the horizontal direction for any plane across the light pipe. The brightness for the backlight along the horizontal axis is somewhat higher near the center of the backlight and lower near the edges due to collimation of the light by the BEF(trademark) layers. By changing the dot density, the brightness can be altered over portions of the backlight exit surface.
A problem with these standard backlight constructions is that changing the dot density not only affects brightness but also reduces the uniformity of light distribution. The silk screen dots also reflect light in scattered directions such that some light is not directed to a desired area or is lost in the system altogether. The overall efficiency of the standard backlight is less than ideal.
Another problem with standard backlight constructions is caused by contact between the separate diffuser film layer and the front surface of the light pipe. No air gap exists between the two since both surfaces are flat and smooth. Both components also typically have very similar refractive indexes. Any light within the light pipe incident on the front surface within the light pipe at a high angle or nearly parallel to the front surface exits the light pipe at the same high angle. The diffuser film and the BEF(trademark) layers cannot completely collimate some of this high angle light back to the normal direction relative to the front surface. This light is either misdirected away from viewers or lost within the system, greatly reducing the efficiency of the backlight.
The present invention is directed to an improved backlight assembly having a light pipe with at least an integral diffuser formed on the back surface of the light pipe. In a separate embodiment, a second integral diffuser is located on the front surface of the light pipe. The present invention simplifies the construction of the backlight assembly and reduces its manufacturing and component cost. The simpler backlight assembly construction of the invention provides improved overall brightness, improved light uniformity, and increased lighting efficiency.
One object of the present invention is to provide a backlight assembly having improved overall brightness characteristics. Another object of the present invention is to provide a backlight assembly with improved light distribution uniformity. A further object of the present invention is to provide a backlight assembly of a simpler construction that is easier to assemble and less costly to manufacture. A further object of the present invention is to provide a backlight assembly having all of these improved characteristics that can be used in place of standard backlight assemblies without any retrofit.
In one embodiment, a backlight assembly of the invention has a pair of light sources disposed along opposite edges of a TIR light pipe. A separate diffuser film layer is disposed adjacent a front surface of the light pipe. A pair of BEF(trademark) enhancing layers brightness are disposed adjacent the diffuser film layer and together collimate light more towards the normal relative to the backlight assembly in both a horizontal and a vertical axis of the backlight. An upper mild diffuser film layer is disposed over the second brightness enhancing film, as an option, to further evenly distribute light exiting the second brightness film, but more importantly, to protect the optical elements of the brightness enhancing film layers. The light pipe also has a back surface opposite the front surface and a reflective surface adjacent the back surface. An integral surface diffuser microstructure is formed on the back surface of the light pipe. In another embodiment, another additional integral surface diffuser microstructure is formed on the front surface of the light pipe. Each integral diffuser surface has particular light distributing and directing characteristics that can vary depending upon the needs of the particular backlight assembly.
In one embodiment, the integral diffuser surface structure formed on the back surface of the light pipe has different light shaping and directing characteristics for different regions of the light pipe. The diffuser is formed so that it directs light in a particular predetermined pattern and wherein the pattern varies for different regions of the light pipe.
In one embodiment, the integral diffuser surface structure on the back surface of the light pipe is formed so that light is emitted in elliptical shaped patterns. In another embodiment, the integral diffuser surface structure is formed so that light is emitted in one elliptical pattern near the light sources and emitted in a different elliptical pattern near the center of the light pipe away from the light sources. In one embodiment, light is emitted from regions near the light sources having about a 95xc2x0 horizontal by about a 15xc2x0 vertical elliptical shape. Light is emitted in the center region having about a 95xc2x0 horizontal by about a 30xc2x0 vertical elliptical shape.
In one embodiment, the integral diffuser surface structure formed on the back surface of the light pipe produces an elliptical shaped light output that does not vary over the surface area of the light pipe. In one embodiment, the diffuser surface structures on the back surface produce an elliptical light output of about 95xc2x0 horizontal by about 25xc2x0 vertical over the entire surface area of the light pipe.
By utilizing the construction of the present invention, and by varying the output characteristics of the various integral diffuser surface structures, a backlight assembly is provided that has greater light distribution uniformity and yet provides improved overall brightness enhancement when compared to prior backlight assembly constructions.
These and other objects, features and advantages of the present invention will become apparent and better understood when considered in conjunction with the following detailed description and accompanying drawings. It should be understood, however, that while indicating preferred embodiments of the present invention, the following description is given only by way of illustration and not of limitation. Many changes and modifications can be made without departing from the scope and spirit of the invention and the invention includes all such modifications.